Pressure-Engineered Structural and Optical Properties of Two

Subscriber access provided by YORK UNIV

Article

Pressure-Engineered Structural and Optical Properties of TwoDimensional (C4H9NH3)2PbI4 Perovskite Exfoliated nm-Thin Flakes Tingting Yin, Bo Liu, Jiaxu Yan, Yanan Fang, Minghua Chen, Wee Kiang Chong, Shaojie Jiang, Jer-Lai Kuo, Jiye Fang, Pei Liang, Su-Huai Wei, Kian Ping Loh, Tze Chien Sum, Timothy J. White, and Ze Xiang Shen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07765 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

ACS Paragon Plus Environment

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Pressure-‐Engineered Structural and Optical Properties of Two-‐Dimensional (C 4 H 9 NH 3 ) 2 PbI 4 Perovskite Exfoliated nm-‐ Thin Flakes Tingting Yin†, Bo Liu∇,○, Jiaxu Yan*,ˆ,○, Yanan Fang , Minghua Chen^, Wee Kiang Chong§,○, Shaojie Jiang‡, Jer-Lai Kuoǁ‖, Jiye Fang‡, Pei Liang∞, Shuhuai Wei⊿, Kian Ping Loh ∇, Tze Chien Sum*,○, Timothy J. White*, , Ze Xiang Shen*,†,○ ⊥

⊥

ˆKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (Nanjing Tech), 30 South Puzhu Road, Nanjing 211816, P. R. China. †

Centre for Disruptive Photonic Technologies, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371. ∇Department

of Chemistry and Centre for Advanced 2D Materials (CA2DM), 3 Science Drive 3,

Singapore 117543. ○

Division of Physics and Applied Physics, School of Physical and Mathematical Sciences (SPMS),

NTU, 21 Nanyang Link, Singapore 637371. ⊥

ERI@N, Research Techno Plaza, X-Frontier Block, Level 5, 50 Nanyang Drive, Singapore 637553.

§

Energy Research Institute @ NTU, ERI@N, Interdisciplinary Graduate School, Nanyang

Technological University, Singapore 639798. ^

Key Laboratory of Engineering Dielectric and Applications (Ministry of Education), Harbin

University of Science and Technology, Harbin 150080, P. R. China. ‡

Materials Science and Engineering Program State University of New York at Binghamton

Binghamton, New York 13902, USA. ǁ‖

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan.

∞

College of Optical and Electronic Technology, China Jiliang University, 310018 Hangzhou, China.

⊿Beijing

Computational Science Research Center, 100094 Beijing, China.

*E-mail: [email protected]; [email protected]; [email protected]; [email protected]

1


Page 2 of 19

Page 3 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Abstract Resolving the structure-property relationships of two-dimensional (2D) organic-inorganic hybrid perovskites is essential for the development of photovoltaic and photoelectronic devices. Here, pressure (0 - 10 GPa) was applied to 2D hybrid perovskite flakes mechanically exfoliated from butylammonium lead halide single crystals, (C4H9NH3)2PbI4, from which we observed a series of changes of the strong excitonic emissions in the photoluminescence spectra. By correlating with in situ high-pressure X-ray diffraction results, we examine successfully the relationship between structural modifications in the inorganic PbI42- layer and their excitonic properties. During the transition between Pbca (1b) phase and Pbca (1a) phase at around 0.1 GPa, the decrease in bond angle and increase in Pb-I bond length lead to an abrupt blue shift of the excitonic bandgap. The presence of the P21/a phase above 1.4 GPa increases the bond angle and decreases the Pb-I bond length, leading to a deep red shift of the excitonic bandgap. The total band gap narrowing of ~350 meV to 2.03 eV at 5.3 GPa before amorphization, facilitates (C4H9NH3)2PbI4 as a much better solar absorber. Moreover, phase transitions inevitably modify the carrier lifetime of (C4H9NH3)2PbI4, where an initial 150 ps at ambient phase is prolongated to 190 ps in the Pbca (1a) phase along with enhanced photoluminescence (PL), originating from pressureinduced strong radiative recombination of trapped excitons.The onset of P21/a phase shortens significantly the carrier lifetime to 53 ps along with a weak PL emission due to pressureinduced severe lattice distorsion and amorphization. High-pressure study on (C4H9NH3)2PbI4 nm-thin flakes may provide insights into the mechanisms for synthetically designing novel 2D hybrid perovskite based photoelectronic devices and solar cells.

2



Page 4 of 19

Introduction Three-dimensional (3D) hybrid perovskites have been extensively studied as light-harvesting materials with power conversion efficiencies (PCE) of up to 23.3 % certified,1 but suffer compromised performance due to hydroscopic decomposition,2-3 ready oxidization,4 heat sensitivity5 and light degradation.6 These limitations are less severe in butylammonium (BA) – methylammonium (MA) two-dimensional (2D) lead iodide perovskites with general formula, (C4H9NH3)2(CH3NH3)n-1PbnI3n+1, or more simply, ((BA)2(MA)n−1PbnI3n+1), where n is the inorganic layer number with 1 ≦ n ≦ ∞.7 The BA+ groups intercalate with ((MA)n−1PbnI3n+1)2− blocks to produce lamellar structures that are stable for at least two months in air8-9 and achieve PCE of 12.52%.9-10 In addition, the direct band gap can be tailored to cover the visible region by adjusting the inorganic layer number n.11 These features render 2D hybrid perovskites superior to their 3D counterparts as tunable and highly efficient light absorbers for optoelectronic12-15 and photovoltaic applications.10, 16-18 Thus, from the point of view of improved photovoltaic operation, it is essential to satisfy a widerange bandgap tunability in 2D layered hybrid perovskites.

The electronic band structure is intimately linked with chemical bonding character that is pressure sensitive, and can be modified by ‘internal’ stress through the modification of relative ionic sizes19 or ‘externally’ through application of strain20 and hydrostatic pressure. The latter approach proves particularly useful to develop a fundamental understanding of electronic structures since the influence of compositional variation is avoided. Moreover, in hybrid perovskites, relatively modest pressures of a few gigapascals (up to 10 GPa), can elicit polymorphic conversions sometimes inaccessible by chemical tuning.21 For 3D perovskites, pressure-induced phase transformations,13,

22-24

amorphization,25 bandgap shifts,13,

increased electronical conductivity,27 and carrier-lifetime prolongation23,

3


28

23, 26

have been

Page 5 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


observed. As for 2D hybrid perovskites, only one prelimiminary high-pressure study on (C4H9NH3)2PbI4 (BAPI) bulk crystals in 2004,29 reported the pressure-driven change of the excitonic band edge in the absorption spectra without any structural analysis and physical explanation. Recently, pressure-induced two-step red shift in photoluminescence (PL) of (BA)2(MA)Pb2I7 (n=2) bulk crystals was reported and explained by the layer-to-layer and the interlayer compression under high pressure, without paying close attentions to phase transitions.30 Most recently, one paper reported that an enduring bandgap narrowing (8.2 % decrease) was achieved in (BA)2(MA)2Pb3I10 (n=3) bulk crystals after compressiondecompression cycles.31 Until now, complete crystal structures under high pressure fail to be resolved and the pressure-dependent evolution of the direct excitonic transitions cannot be obtained by using thick samples. Therefore, a full assessment of the crystal structures of 2D hybrid perovskites under high pressure will shed light on the structure-excitonic property correlation and deepen the fundamental understanding of the crystallochemical drivers of excitonic properties for their potential applications.

In this work, correlations between pressure-mediated crystal structural adjustments and excitonic properties were examined to the first time in the BAPI (n=1) 2D perovskite exfoliated flakes using in situ synchrotron powder X-ray diffraction (XRD), Raman spectroscopy and PL spectroscopy supported with ab initio calculations. As for the excitonic absorption spectra, we have chosen the thinner exfoliated BAPI flakes with thickness of 50 nm. We found that bandgap shifts under pressure are controlled by Pb and I orbital interactions reflected both in the Pb-I bond length within a PbI6 octahedron and the bond angle between two PbI6 octahedra, where the decrease of Pb-I bond length and bond angle modifies the bandgap in an opposite sense. In addition, the bond angle (octahedral tilting) is the dominant structural factor impacting the band gap across

4



polymorphic transitions, while the Pb-I bond length (contraction) is the major influence on the excitonic structure within each single phase. The structure-property relationship established here provides general guidance for engineering the excitonic character of 2D hybrid perovskites. By choosing proper M/X element combinations that deliver less distorted MX42- lattices and shorter M-X bond lengths, 2D layered hybrid perovskites with smaller band gaps are produced especially for solar cell applications.

Results and Discussion Solution grown BAPI (n=1) perovskites appear as orange yellow bulk polycrystals, which limit the optical characterizations of intrinsic excitonic properties. So thin flakes of BAPI (Figure 1a) were exfoliated by following the method developed for graphene exfoliation,32 and transfered onto a culet of 500 µm of the diamond anvile cell (DAC) (Figure 1b) for in situ high-pressure optical analyses. Since the thin flakes of BAPI sample is air sensitive, the hexagonal boron nitride (hBN) thin layer was also transfered onto the culet to cover the BAPI thin flakes to prevent direct air contact.33 Alignment of the transfered BAPI flakes and hBN layer was performed under a microscope in the glovebox, a controlled argon environment. The sample thickness determination is shown in Figure S1. Upon compression from 0 to 10 GPa, the BAPI displays piezochromic transitions (Figure 1c), indicating pressure-driven shifts of absorption edge in BAPI (Figure 1d). The strong absorption peaks confirm the intense direct excitonic transitions. More important, we observed a series of changes of the strong excitonic emissions in the PL spectra, where the initial PL peak I of BAPI at ambient condition is 524 nm (2.37 eV), then red shifts a little bit to 527 nm (2.35 eV) with an additonal peak II at 503 nm (2.47 eV) at ~0.14 GPa, suggesting the appearance of the second phase. With increasing pressure, peak II keeps a consistant red shift until another new

5


Page 6 of 19

Page 7 of 19

emission peak III grows up at ~540 nm (2.30 eV) at 1.4 GPa, indicating the third phase. With further compression, the peak III becomes Pressure

(a)

(b)

(c)

0.4 GPa

0.6 GPa

1.9 GPa

7.3 GPa

20 um

ruby

50 um (C 4H9NH3)2PbI4

Pressure

(d) 3.9 GPa 3.2 GPa 2.6 GPa 2.4 GPa 1.9 GPa 1.4 GPa 1.2 GPa 0.6 GPa 0.4 GPa 0 GPa

(e)

Photoluminescence (a.u.)

5.3 GPa

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5.3 GPa

III

II

480

450 480 510 540 570 600 630 Wavelength (nm)

I

510 540 570 600 Wavelength (nm)

3.9 GPa 3.0 GPa 2.6 GPa 2.4 GPa 1.9 GPa 1.4 GPa 1.2 GPa 0.6 GPa 0.4 GPa 0.14 GPa 0 GPa

630

Figure 1. Optical characterizations of BAPI (BA = C4H9NH3+) 2D layered perovskite single crystal under high pressure. (a,b) The optical image of the original BAPI exfoliated flakes inside the DAC. (c) Optical micrographs at selected pressures showing piezochromism in BAPI. (d,e) In situ high-pressure absorption and PL spectra of the BAPI exfoliated flakes. dominant and exhibits a consistant red shift with a pronounced band gap narrowing down to 611 nm (2.03 eV) at 5.3 GPa. At 10 GPa, both the absorption and PL spectra become very broad, which is obvious that BAPI crystals transform into the amorphous phase at very high pressure. After pressure released back to 0 GPa, the PL emission was reversible (the blue line in Figure S2b), as demonstrated by the diffraction pattern (the top black line in Figure 2a) and the Raman spectra (the top lines in Figure 2c) measured in the pressure-released BAPI sample.

6



Pressure-mediated crystal structures. Structural variation is the main origin for the intriguing excitonic band change under high pressure. Figure 2a shows the in situ highpressure powder X-ray diffraction patterns obtained from BAPI powder sample. At ambient conditions, the BAPI polymorph is orthorhombic Pbca (1b), i.e., room-temperature (RT) phase,34-35 with a = 8.851(2) Å; b = 8.654(3) Å; c = 27.735(3) Å (Figure S3a and Table S1), where the PbI42- sheets are separated by interdigitated bilayers of butylammonium (BA) cations (Figure 2b).34 The inorganic-organic layers communicate through hydrogen-bonding between the NH3+ groups and the I- ions.36-37 From the structural top view, the NH3+ group faces to the obtuse angle of the parallegoram defined by the four bridging I (Figure S4a). The first phase transition starts from ~0.1 GPa (Figure S5), the pure new structure shows abrupt displacements for Bragg (002) and (111) reflections and the interchange of Bragg (200) and (020) reflections (Figure 2a). Based on the structural refinement, we identify that this new phase still maintans Pbca (1b) space symmetry but swapping a and b axis, with a = 8.484(1) Å; b = 9.111(1) Å; c = 27.128(4) Å (Figure S3b and Table S1), i.e., Pbca (1a). This space group is consistent with that low-temperature (LT) phase reported in previous temperaturedependent differential scanning calorimetry (DSC) and XRD measurements on the BAPI single crystals.35 Since cooling and compression have the similar effects on the structural symmetry of crystals, we name this high-pressure induced new phase, Pbca (1a), as ‘LT’ phase in this paper. In the Pbca (1b) RT phase, PbI6 octahedral tilting is entirely in the a-b plane, while the out-of-plane tilting leads to a large change in the a and b,38 and the NH3+ group faces to the acute angle of the parallegoram defined by the four bridging I (Figure S4b). At ~1.6 GPa, the emergence of Bragg (002), (111) and (020) reflections appear at ~2.15º, 4.45º and 6.09º, indicates a lower symmetry. The diffraction pattern can be fitted well by the monoclinic P21/a space group with a = 8.500(3) Å; b = 9.298(6) Å; c = 12.673(5) Å; β 7


Page 8 of 19

Page 9 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


=103.48(1)º that coexists with the Pbca (1a) polymorph (Figure S3c and Table S1). Above 5.0 GPa, the transformation to the high-pressure (HP) P21/a

8



(a)

(002)

(020)

(200)

(111)

(b) HP

HP

c (001)

(111)

(001)

a

(020)

LT+HP

LT

(002)

(111)

(001)

c

(020)

(020)

(111)

a

LT

(002)

0.4

RT

(020)

(111)

(002)

(200)

(111)

(020) c

Packing diagram

(c)

D0 GPa 7.0 GPa 5.6 GPa C-‐N stretching

BA breathing W2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 19

41 cm-‐1

5.1 GPa 4.4 GPa 3.3 GPa 2.6 GPa 2.0 GPa 1.7 GPa

HP

BA rocking 737 cm-‐1

23 cm-‐1 50 cm-‐1

1.2 GPa 0.9 GPa 0.6 GPa 0.4 GPa 0.15 GPa 0 GPa

-‐60

LT+HP

W3

W1

-‐90

856 cm-‐1

501 cm-‐1

LT

RT -‐30

30

60

500

90

600

Raman shift (cm-‐1) 9


700

800

900

a

Page 11 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Structural characterizations of BAPI under high pressure. (a) Integrated in situ high-pressure synchrotron powder XRD patterns of BAPI at three representative 2 theta ranges. (b) Schematic illustrations of three crystal structures of BAPI under high pressure: organic- inorganic packing diagrams with interdigitating bilayers of BA spacer cations being inserted between PbI42- inorganic sheets. (c) Representative vibrational modes of inorganic PbI6 octahedra and BA organic molecules under pressure. Black, purple, brown, blue and light pink spheres represent Pb, I, C, N and H atoms, respectively. H atoms are omitted in packing diagram of the crystal structure for clarity. phase has completed (Figure S3d), and is characterized by the monoclinic angle (β) (Table S1) due to the movement of every second layer of the LT phase by b/2 to realize an eclipsed arrangement of the adjacent inorganic layers (Figure 2b), that efficiently halves the c parameter (Table S1). These two phase transitions are confirmed from the Raman spectra in the low-frequency region (Figure 2c), due to the inorganic octahedral in-phase (W1) and out-of-phase bending (W3) modes becoming remarkably sharp in the Pbca (1a) LT phase with an additional octahedral out-of-phase rotation (W2) mode appearing in the P21/a HP phase.39-40 During octahedral tilting, the BA molecule (Figure S4), reorientates from almost parallel to the long diagonal of the parallelogram formed by the adjacent four bridging I atoms in Pbca (1b) RT phase, to the short diagonal in Pbca (1a) LT and P21/a HP phase. The orientation of the BA molecule is directly revealed by the representative Raman modes in high-frequency region (Figure 2c),40-41 where the BA rocking mode is sharp in Pbca (1a) LT phase, while the BA breathing and C-N stretching modes are active only in P21/a HP phase. All the Raman shifts are reversible upon decompression (the top lines in Figure 2c), demonstrating that [C4H9NH3]+ chains act as the spring cushion to protect the PbI42- layers and the whole structure.24, 42 Pressure-mediated excitonic structures. To fully understand the interplay between crystal structures and excitonic bands under high pressure, a series of theoretical calculations are performed. Firstly, we compared the excitonic band structures at 0 GPa, 0.4 GPa and 3.0 GPa, 10



three key pressure points corresponding to Pbca (1b) RT, Pbca (1a) LT and P21/a HP pure phases (Figure 3a, Figure 3b and Figure 3c). It is clear that the band gap is increased up to 2.16 eV at 0.4 GPa compared to the value of 1.97 eV at 0 GPa while is decreased to 1.96 eV at 3.0 GPa, which correlates to the blue shift for the first phase transition and a red shift for the second phase transition observed in both measured absorption and PL spectra. Then, we calculated a series of bandgap energies (the circles in Figure 3d) and plotted them as a function of pressure to compare with the experimental excitonic peak positions (the spheres in Figure 3d). The pressure dependency of the band gaps for LT and HP phases was evaluated with respect to two distinct structural effects: bond angle (Figure 3e) and Pb-I bond length (Figure 3f). At 0.4 GPa, the bond angle undergoes a sudden drop from 150.2º to 144.9º and the elongated averaged equatorial Pb-I bond length enlongates dramatically from 3.09Å to ~3.14Å (Table S2), corresponding to a transition from Pbca (1b) RT – Pbca (1a) LT phase. The more acute bond angle and longer Pb-I bond length diminish Pb-I orbital overlap, weakening antibonding interactions of the Pb 6s and the I 5px/py orbitals and reducing the valence band maximum (VBM). Simultaneously, the conduction band minimum (CBM) mainly formed by Pb 6p orbitals keeps almost no shift (Figure 3a and Figure 3b). Overall, the band gap becomes broad and a blue shift is observed at 0.4 GPa in both absorption and PL spectra (Figure 1d and Figure 1e). The anomalous increase of the bond length under compression is due to structural symmetry change in Pbca (1a) LT phase, where the reorientation of long-chain BA molecules and the out-of-plane bended distortion of inorganic PbI6 octahedra may make Pb-I bond more space to release (Figure 2b). The onset of the P21/a HP phase at ~1.5 GPa, the bond angle increases to 150.5º and the averaged Pb-I bond length further decreases to 3.09Å (Table S2), both contributing a more narrowed band gap and a second peak in both absorption and PL spectra (Figure 1d and Figure 1e). Such an increase of the bond angle originates

11


Page 12 of 19

Page 13 of 19

from the more intersected long-chain BA molecules under higher pessure and the in-plane twisted inorganic PbI6 octahedra (Figure 2b).

(d) 2.7

(c) 9

8

8

8

7 1.97 eV

7 2.16 eV

7 1.96 eV

6

6

6

0 GPa Γ Μ R X Γ

VBM

0.4 GPa Γ Μ R X Γ

3.0 GPa

RT

2.4

HP

2.1 1.8

Γ Μ R X Γ

0

156 α

150

RT LT HP

144 0

1 2 3 4 5 Pressure (GPa)

(f)

Bond length (Å)

(e)

I1

3.06

Pb

I2

RT LT HP

3.00 0

1

2 3 4 Pressure (GPa)

5

(g)

3.18 averaged Pb-‐I 3.12

RT LT HP

LT

Band gap (eV)

(b) 9

Energy (eV)

Energy (eV)

(a)CBM 9

Bond angle (º)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.2 2.0

LT RT HP

1.8

Pb-‐I VS Eb VS Eb

1.6


0


Figure 3. Correlation of structure-excitonic property of BAPI under high pressure. (a-c) Calculated band structures and the isosurfaces of partial electron densities for VBM and CBM of BAPI under representative pressures (0 GPa, 0.4 GPa and 3.0 GPa). (d) Exciton evolution as a function of pressure: experiment (spheres) and calculation (circles), respectively. The colourful shallows represent phase evolution with increasing pressure. Derived structural parameters of bond angle (e) and the averaged value of two equatorial Pb-I bond lengths (f) from the structural models used for bandgap calculations in (d). (g) Band gaps calculated by purely changing the bond angle (diamonds) or changing the bond length (pentagons). For LT and HP phases, we adopted the optimized atomic structure at 0 and 1.5 GPa as the initial model, respectively. To determine quantitatively the effect of two structural factors on the excitonic band gap in each single phase. The calculated bandgap energy as a function of the bond angle (the wine red diamonds) and Pb-I bond length (the ink blue diamonds) is plotted in Figure 3g. Wthin the Pbca (1a) LT phase from 0.4 to 3.0 GPa, the decrease in bond angle (Δ=-1.21º) 12



and bond length (Δ=-0.071Å) modifies the bandgap in an opposite sense, as the bond length narrows the energy bandgap Δ=-0.23 eV and the bond angle widdens the bandgap only Δ=0.08 eV (the ink blue diamonds in Figure 3g), resulting in an overall red shift in the spectroscoptic data in Figure 3d. Within the P21/a HP phase from 1.4 to 5.3 GPa, the increase of bond angle (Δ=3.75º) and further decreased bond length (Δ=-0.087Å) contribute a consistent action in bandgap narrowing (the wine red pentagons in Figure 3g), resulting in a more pronounced red shift in the observed excitonic peaks compared to the Pbca (1a) LT phase. Our comprehensive calculations fully resolve the structural origin for the observed blue jump and red drop during phase transitions and two-regime red-shift behavior of the excitonic emission peaks in two distinct phases, providing a synthetic method for fine-tuning the energy band structure.

Pressure-mediated exciton dynamics. Changes in crystal structures may further engineer carrier properties, as explored by high-pressure time-resolved photoluminescence (TRPL) (Figure 4a, Figure 4b and Figure 4c). At ambient pressure, the mean exciton lifetime is ~150 ± 20 ps, consistent with the reported timescale for n=1 BAPI single crystals,12, 43 where the fast decay channel (τ1=51 ± 1 ps) refers to the recombination of free excitons and the slow decay channel (τ2=320 ± 10 ps) refers to the recombination of localized trapped excitons. It can also be demonstrated by the asymmetric line-shape of PL spectra (Figure 1e), the long tail comes from radiative recombination of excitonic trap states.44 A longer exciton lifetime of ~190 ± 10 ps is observed at 0.4 GPa Pbca (1a) LT phase, where the lifetime of trapped excitons is prolongated to 440 ± 20 ps, indicating that more excitons are trapped by the distorted PbI42- lattices and localized at the organic-inorganic interfaces of BAPI.23 This trapassisted excitonic radiative emission process contributes to enhanced PL emission in Pbca (1a) LT phase with a maximum value of 2.3 (Figure 4d). The presence of the P21/a HP phase 13


Page 14 of 19

Page 15 of 19

leads to a significant shortening in carrier lifetime, indicating a strong nonradiative recombination process due to more and more defects introduced by pressure-induced mixed phase and the onset of amorphization. For example, the carrier lifetime becomes ~36 ± 5 ps at 2.3 GPa, where the lifetime of both free and trapped PL decay

PL (norm.)

1

Total fit

Fast component τ1 (b)

RT phase

0 GPa

1

= 150 ± 20 ps

0.1 0.01 τ2 = 320 ± 10 ps 1E-‐3 τ1 = 51 ± 1 ps

= 53 ± 7 ps

0.1

0.01

τ2 = 440 ± 20 ps

0

τ2 = 148 ± 5 ps 1E-‐3 τ1 = 19 ± 1 ps

300 600 Time (ps)

900

300 600 Time (ps)

(d)

200.0k

50.0k

900 RT LT HP

LT + HP

LT

100.0k

0.01

0

= 190 ± 10 ps

150.0k

ooooo o ooo o

LT phase

0.4 GPa

1E-‐3 τ1 = 39 ± 1 ps

Integrated PL intensity (cts.)

1

Slow component τ2

0.1

0

(c)

300 600 900 Time (ps) LT+HP phase 2.3 GPa

PL (norm.)

(a)

PL (norm.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RT

HP

0.0 0

1 2 3 4 Pressure (GPa)

5

Figure 4. Carrier lifetime in high pressure-induced three phases. (a-c) TRPL spectra of BAPI single crystal at 0 GPa (RT phase), 0.4 GPa (LT phase), and 2.3 GPa (LT+HP phase). All the TRPL spectra were obtained from the peak I in static PL spectra. A biexponential treatment (IPL (t) = Iint [Aslow exp (-t/τslow) + Afast exp (-t/τfast) + I0]) used to extract the mean carrier time ( = [Aslow τ2slow / (Aslow τslow + Afast τfast)] + [Afast τ2fast / (Aslow τslow + Afast τfast)]), where τslow and τfast are assigned to the trapped and free exciton recombination respectively. (d) Pressure dependence of integrated PL intensities for the corresponding peaks from 0 to 5.3 GPa. PL emission is enhanced in Pbca (1a) LT phase with a maximum enhancement of 2.3 times at 0.4 GPa. excitons shortens significanly to 16 ± 1 ps and 153 ± 6 ps, respectively. The second PL peak is dominated from these self-trapped excitions, as demonstrated by the relative fat and symmetric PL spectra under higher pressure (Figure 1e). The PL emission intensity in P21/a HP phase (the wine red cubes in Figure 4d) becomes even lower than the initial PL intensity 14



(the black cube in Figure 4d) just due to the nonradiative decay of self-trapped excitions. Further high-pressure time-resolved characterizations assist the physical understanding of the high-pressure effect on photoelectric properties.

Conclusions The straightforward relationship between crystallographic polymorphsim and excitonic behaviors has been resolved for 2D hybrid perovskites via the comprehensive high-pressure characterizations, by optically monitoring the evolution of strong excitonic peaks in exfoliated flakes and carefully refining the structural variation supported by a series of ab initio calculations. Smaller bond angles widen the band gap, while shorter Pb-I bond lengths have the opposite effect. The two-step redshift behavior in excitionic bandgap of BAPI 2D perovskite will lead to superior solar absorbers. High pressure is an effective tool to provide deeper insights into structure-property correlations from the atomic level, to inform the optimization of functional properties (e.g., high-level bandgap narrowing and carrier lifetime prolongation / shortening) through the synthetic design of hybrid perovskite materials for photovoltaic and photoelectric applications.

Author Information Corresponding Authors *[email protected]

*[email protected] *[email protected] *[email protected] Author Contributions T.T. Y (Dr.), B.L (Dr.), J.X.Y (Dr.) and Y.N.F (Dr.) contributed equally. 15


Page 16 of 19

Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Notes The authors declare no competing financial interests.

Acknowledgements T.T. Y, J.X. Y and Z.X.S gratefully acknowledge the Ministry of Education (MOE) for the following grants: AcRF Tier 1 (Reference No: RG103/16); AcRF Tier 2 (MOE2015-T2-1148); AcRF Tier 3 (MOE2011-T3-1-005). J.X.Y is supported by the National Natural Science Foundation of China (Grant No. 11704185) and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK20171021). T.C.S. receives funding from the Ministry of Education Academic Research Fund Tier 1 Grant RG173/16, Tier 2 Grants MOE2015-T22-015 and MOE2016-T2-1-034, and from the Singapore (NRF) through the Singapore– Berkeley Research Initiative for Sustainable Energy (SinBeRISE) CREATE Program and the Competitive Research Program NRF-CRP14-2014-03. S.H.W. is supported by the National Key Basic Research Program of China (2016YFB0700700) and National Natural Science Foundation of China (51672023, 11634003, U1530401). S.J. and J. F. thank Dr. Zhongwu Wang and Dr. Ruipeng Li for their assistance and acknowledge the support from Custom Electronics Inc. and Binghamton University. CHESS was supported by the NSF award DMR1332208.

Supporting Information Available Synthesis process of thin layer (C4H9NH3)2PbI4; In situ high-pressure experimental details of XRD measurements, static absorption/PL/Raman measurements and time-resolved PL measurements; Ab initio calculations; Crystallographic information; Additional results, and References. This material is available free of charge via the Internet at http://pubs.acs.org/. 16



References (1) NREL. Best Research-Cell E ciencies, http://www.nrel.gov/ncpv/images/efficiency_chart.jpg (accessed 2018). (2) Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; Docampo, P.; Barnes, P. R. F. Chem. Mater. 2015, 27, 3397-3407. (3) Lee, J. W.; Kim, D. H.; Kim, H. S.; Seo, S. W.; Cho, S. M.; Park, N. G. Adv. Energy Mater. 2015, 5, 201501310. (4) Aristidou, N.; Sanchez-Molina, I.; Chotchuangchutchaval, T.; Brown, M.; Martinez, L.; Rath, T.; Haque, S. A. Angew. Chem. Int. Ed. 2015, 54, 8208-8212. (5) Misra, R. K.; Aharon, S.; Li, B. L.; Mogilyansky, D.; Visoly-Fisher, I.; Etgar, L.; Katz, E. A. J. Phys. Chem. Lett. 2015, 6, 326-330. (6) Niu, G.; Guo, X.; Wang, L. J. Mater. Chem. A 2015, 3, 8970-8980. (7) Quan, L.; Yuan, M.; Comin, R.; Voznyy, O.; Beauregard, E. M.; Hoogland, S.; Buin, A.; Kirmani, A. R.; Zhao, K.; Amassian, A.; Kim, D. H.; Sargent, E. H. J. Am. Chem. Soc. 2016, 138, 2649-2655. (8) Saparov, B.; Hong, F.; Sun, J.; Duan, H.; Meng, W.; Cameron, S.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Chem. Mater. 2015, 27, 5622-5632. (9) Jaffe, A.; Lin, Y.; Mao, W.; Karunadasa, H. I. J. Am. Chem. Soc. 2015, 137, 1673-1678. (10) Tsai, H.; Nie, W.; Blancon, J.-C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G.; Mohite, A. D. Nature 2016, 536, 312-316. (11) Yuan, M.; Quan, L.; Comin, R.; Walters, G.; Sabatini, R.; Voznyy, O.; Hoogland, S.; Zhao, Y.; Beauregard, E. M.; Kanjanaboos, P.; Lu, Z.; Kim, D. H.; Sargent, E. H. Nat. Nanotechnol. 2016, 11, 872-877. (12) Guo, Z.; Wu, X.; Zhu, T.; Zhu, X.; Huang, L. ACS Nano 2016, 10, 9992-9998. (13) Wang, N.; Cheng, L.; Ge, R.; Zhang, S.; Miao, Y.; Zou, W.; Yi, C.; Sun, Y.; Cao, Y.; Yang, R.; Wei, Y.; Guo, Q.; Ke, Y.; Yu, M.; Jin, Y.; Liu, Y.; Ding, Q.; Di, D.; Yang, L.; Xing, G.; Tian, H.; Jin, C.; Gao, F.; Friend, R.; Wang, J.; Huang, W. Nat. Photonics 2016, 10, 699-704. (14) Liu, W.; Xing, J.; Zhao, J.; Wen, X.; Wang, K.; Lu, P.; Xiong, Q. Adv. Opt. Mater. 2017, 5, 1601045. (15) Mao, L.; Wu, Y.; Stoumpos, C. C.; Wasielewski, M. R.; Kanatzidis, M. G. J. Am. Chem. Soc. 2017, 139, 5210-5215. (16) Grancini, G.; Roldán-Carmona, C.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M.; Nazeeruddin, M. K. Nat. Commun. 2017, 8, 15684. (17) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. J. Am. Chem. Soc. 2015, 137, 7843-7850. (18) Cohen, B.-E.; Wierzbowska, M.; Etgar, L. Adv. Funct. Mater. 2017, 27, 1604733. (19) Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F. Nat. Commun. 2014, 5, 5757. (20) Zhao, J.; Deng, Y.; Wei, H.; Zheng, X.; Yu, Z.; Shao, Y.; Shield, J. E.; Huang, J. Sci. Adv. 2017, 3, eaao5616. (21) Tan, J. C.; Cheetham, A. K. Chem. Soc. Rev. 2011, 40, 1059-1080. (22) Jiang, S.; Fang, Y.; Li, R.; Xiao, H.; Crowley, J.; Wang, C.; White, T. J.; Goddard, W. A.; Wang, Z.; Baikie, T.; Fang, J. Angew. Chem. Int. Ed. 2016, 55, 6540-6544. (23) Xiao, G.; Cao, Y.; Qi, G.; Wang, L.; Liu, C.; Ma, Z.; Yang, X.; Sui, Y.; Zheng, W.; Zou, B. J. Am. Chem. Soc. 2017, 139, 10087-10094. (24) Yin, T.; Fang, Y.; Kiang, C. W.; Koh, T. M.; Jiang, S.; Li, X; Kuo, J-L; Fang, J.; Sum, T-C.; White, T. J.; Yan, J.; Shen, Z. Adv. Mater. 2018, 30, 1705017. (25) Wang, Y.; Lu, X.; Yang, W.; Wen, T.; Yang, L.; Ren, X.; Wang, L.; Lin, Z.; Zhao, Y. J. Am. Chem. Soc. 2015, 137, 11144-11149. 17


Page 18 of 19

Page 19 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Jaffe, A.; Lin, Y.; Beavers, C. M.; Voss, J.; Mao, W. L.; Karunadasa, H. I. ACS Cent Sci. 2016, 2, 201-209. (27) Jaffe, A.; Lin, Y.; Mao, W.; Karunadasa, H. I. J. Am. Chem. Soc. 2017, 139, 4330-4333. (28) Kong, L.; Liu, G.; Gong, J.; Hu, Q.; Schaller, R. D.; Dera, P.; Zhang, D.; Liu, Z.; Yang, W.; Zhu, K.; Tang, Y.; Wang, C.; Wei, S.; Xu, T.; Mao, H.-K. Proc. Natl. Acad. Sci. U.S.A. 2016, 113, 8910-8915. (29) Matsuishi, K.; Ishihara, T.; Onari, S.; Chang, Y. H.; Park, C. H. Physica status solidi B 2004, 241, 3328-3333. (30) Liu, G.; Kong, L.; Guo, P.; Stoumpos, C. C.; Hu, Q.; Liu, Z.; Cai, Z.; Gosztola, D. J.; Mao, H.-k.; Kanatzidis, M. G.; Schaller, R. D. ACS Energy Lett. 2017, 2, 2518-2524. (31) Liu, G.; Gong, J.; Kong, L.; Schaller, R. D.; Hu, Q.; Liu, Z.; Yan, S.; Yang, W.; Stoumpos, C. C.; Kanatzidis, M. G.; Mao, H.-K.; Xu, T. Proc. Natl. Acad. Sci. U.S.A. 2018, 115, 8076-8081. (32) Andres, C.-G.; Michele, B.; Rianda, M.; Vibhor, S.; Laurens, J.; Herre, S. J. v. d. Z.; Gary, A. S. 2D Mater. 2014, 1, 011002. (33) Abdelwahab, I.; Grinblat, G.; Leng, K.; Li, Y.; Chi, X.; Rusydi, A.; Maier, S. A.; Loh, K. P. ACS Nano 2018, 12, 644-650. (34) Yin, J.; Li, H.; Cortecchia, D.; Soci, C.; Bredas, J. L. ACS Energy Lett. 2017, 2, 417-423. (35) Billing, D. G.; Lemmerer, A. Acta Crystallogr. Sect. B 2007, 63, 735-747. (36) Yin, T.; Fang, Y.; Fan, X.; Zhang, B.; Kuo, J.-L.; White, T. J.; Chow, G. M.; Yan, J.; Shen, Z. Chem. Mater. 2017, 29, 5974-5981. (37) Egger, D. A.; Kronik, L.; Rappe, A. M. Angew. Chem. Int. Ed. 2015, 54, 12437-12441. (38) Glazer, A. Acta Crystallogr. Sect. B 1972, 28, 3384-3392. (39) Quarti, C.; Grancini, G.; Mosconi, E.; Bruno, P.; Ball, J. M.; Lee, M. M.; Snaith, H. J.; Petrozza, A.; Angelis, F. D. J. Phys. Chem. Lett. 2014, 5, 279-284. (40) Leguy, A. M. A.; Goni, A. R.; Frost, J. M.; Skelton, J.; Brivio, F.; Rodriguez-Martinez, X.; Weber, O. J.; Pallipurath, A.; Alonso, M. I.; Campoy-Quiles, M.; Weller, M. T.; Nelson, J.; Walsh, A.; Barnes, P. R. F. Phys. Chem. Chem. Phys. 2016, 18, 27051-27066. (41) Glaser, T.; Müller, C.; Sendner, M.; Krekeler, C.; Semonin, O. E.; Hull, T. D.; Yaffe, O.; Owen, J. S.; Kowalsky, W.; Pucci, A.; Lovrinčić, R. J. Phys. Chem. Lett. 2015, 6, 2913-2918. (42) Li, B.; Bian, K.; Zhou, X.; Lu, P.; Liu, S.; Brener, I.; Sinclair, M.; Luk, T.; Schunk, H.; Alarid, L.; Clem, P. G.; Wang, Z.; Fan, H. Sci. Adv. 2017, 3, e1602916. (43) Wu, X.; Trinh, M. T.; Zhu, X. J. Phys. Chem. C 2015, 119, 14714-14721. (44) Wu, X.; Trinh, M. T.; Niesner, D.; Zhu, H.; Norman, Z.; Owen, J. S.; Yaffe, O.; Kudisch, B. J.; Zhu, X. Y. J. Am. Chem. Soc. 2015, 137, 2089-2096.

18



For Table of Contents Only

2.6

Band gap (eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2.4

Pb-‐I

(BA)2PbI4

Pbca (1a)

2.2 2.0

P21/a

1.8 Pbca (1b) 0

1

2 3 Pressure (GPa)

4

5

19


Page 20 of 19

Pressure-Engineered Structural and Optical Properties of Two

Recommend Documents